Photometer having a long lamp life, reduced warm-up period and resonant frequency mixing

Abstract

An apparatus and method for making a photometric determination regarding the contents of a sample, such as the absorbance characteristic of a blood serum sample. The improvement allows measurements to be taken after a reduced warm-up period and compensates for variations in the lamp intensity during the period over which measurements are taken. A feedback circuit is provided between a reference photodetector and the light source for causing the light source intensity to asymptotically approach a final stable value in a smooth exponential curve. Signal processing means processes outputs of the reference and sample photodetectors taken at different times so as to compensate for variations in the source intensity over the time period during which the outputs are taken. Still further, the photometer allows resonant frequency mixing of the contents of a cuvette while disposed in the photometer by causing the entire optical assembly, in which the cuvette is mounted, to vibrate at its resonant frequency. A microprocessor-controlled search routine is used to determine the resonant frequency of the optical assembly which is easily recalculated each time there is a change in the dimensions or mass of the optical assembly, or in the contents of the cuvette.

What is claimed is: 
     
       1. In an apparatus for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photodetector for making a photometric determination regarding the contents of the sample, the improved apparatus allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, wherein the improvement comprises: means for stabilizing the light emitted by the source to cause the source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve, wherein the stabilizing means is a feedback circuit between the reference photodetector and the light source having a time constant of from about 0.1 seconds to about 0.3 seconds; and   means for processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and which compensates for variations in the light source intensity over the time period during which the outputs are taken.   
     
     
       2. In an apparatus for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photodetector for making a photometric determination regarding the contents of the sample, the improved apparatus allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, wherein the improvement comprises: means for stabilizing the light emitted by the source to cause the source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve, wherein the stabilizing means is a feedback circuit between the reference photodetector and the light source and stabilizes the intensity of light emitted by the source to within about 1% of less of its final stable value within about 3 seconds or less;   means for processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and which compensates for variations in the light source intensity over the time period during which the outputs are taken.   
     
     
       3. In an apparatus for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photodetector for making a photometric determination regarding the contents of the sample, the improved apparatus allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, wherein the improvement comprises: means for stabilizing the light emitted by the source to cause the source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve; and   means for processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and which compensates for variations in the light source intensity over the time period during which the outputs are taken, wherein the processing means comprises:   means for receiving a series of four outputs taken at different times, a first pair of outputs being the outputs of the reference photodetector taken at equal times before and after a midpoint in time, a second pair of outputs being the outputs of the sample photodetector taken at equal times before and after the midpoint;   means for averaging the first pair of outputs to produce an average reference photodetector measurement and averaging the second pair of outputs to produce and average sample photodetector measurement; and   means for taking a ratio of the average sample photodetector measurement and the average reference photodetector measurement to determine the absorbance characteristic of the sample as if the sample and reference photodetector measurements were taken at the same midpoint in time.   
     
     
       4. In an apparatus for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photodetector for making a photometric determination regarding the contents of the sample, the improved apparatus allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, wherein the improvement comprises: means for stabilizing the light emitted by the source to cause the source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve; and   means for processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and which compensates for variations in the light source intensity over the time period during which the outputs are taken;   wherein the light source is a tungsten filament disposed within a bulb, the bulb being filled with a gas which is unreactive with the tungsten filament and which prevents deposition of the tungsten on the interior of the bulb.   
     
     
       5. The apparatus of claim 4, wherein the gas is selected from the group consisting of krypton and argon. 
     
     
       6. In an apparatus for making a photometric determination regarding the contents of a sample, wherein light from a source is sent to a filter for alternatively selecting one of a plurality of predetermined wavelengths, the light passing through the filter being separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photometer for making a photometric determination regarding the contents of the sample, the improved apparatus allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, wherein the improvement comprises: the light source comprises a tungsten filament disposed within a bulb, the bulb being filled with a gas which is unreactive with the tungsten filament and which prevents deposition of the tungsten on the interior of the bulb;   means connecting the reference photodetector and source for stabilizing the light emitted by the source to cause the source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve for any one of the plurality of predetermined wavelengths selected by the filter; and   means for processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and which compensates for variations in the light source intensity over the time period during which the outputs are taken.   
     
     
       7. In a method for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photodetector for making a photometric determination regarding the contents of the sample, the improved method allowing a photometric measurement to be made after a reduced warm up period and compensating for variations in the intensity of the source during the measurement, the improved method comprising the steps of: turning on the light source;   sending the output of the reference photodetector to a control circuit connected to the source for causing the light source intensity to asymptotically approach a final stable value in a smooth exponential curve;   after the light intensity has reached within about 1% of its final stable value, taking a series of four measurements at separate times, a first pair of measurements being the outputs of the reference photodetector taken at equal times before and after a midpoint in time, a second pair of measurements being the outputs of the sample photodetector taken at equal times before and after the midpoint;   averaging the first pair of measurements to produce an average reference photodetector measurement and averaging the second pair of measurements to produce an average sample photodetector measurement; and   taking a ratio of the average sample photodetector measurement and the average reference photodetector measurement to determine the absorbance characteristic of the sample as if the sample and reference photodetector measurements were taken at the same midpoint in time.   
     
     
       8. In a method for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photo-detector for making a photometric determination regarding the contents of the sample, the improved method allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, the improved method comprising the steps of: turning on the light source;   causing the light source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve;   processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and compensating for variations in the source intensity over the time period during which the outputs are taken;   wherein the light source comprises a tungsten filament disposed within a bulb, the bulb being filled with a gas which is unreactive with the tungsten filament and which prevents deposition of the tungsten on the interior of the bulb.   
     
     
       9. In a method for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photo-detector for making a photometric determination regarding the contents of the sample, the improved method allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, the improved method comprising the steps of: turning on the light source;   causing the light source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve;   processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and compensating for variations in the source intensity over the time period during which the outputs are taken;   wherein the source intensity is caused to asymptotically approach a final stable value by providing a feedback circuit connecting the source and reference photodetector having a time constant of from about 0.1 seconds to about 0.3 seconds.   
     
     
       10. In a method for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through the sample being sent to a sample photo-detector for making a photometric determination regarding the contents of the sample, the improved method allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, the improved method comprising the steps of: turning on the light source;   causing the light source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve;   processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and compensating for variations in the source intensity over the time period during which the outputs are taken;   wherein the intensity of light emitted by the source is stabilized to within about 1% or less of its final stable value within about three seconds or less.   
     
     
       11. In a method for making a photometric determination regarding the contents of a sample, wherein light from a source of a predetermined wavelength is separated into a first portion which is sent to a reference photodetector and a second portion which is sent to a sample, the light passing through sample being sent to a sample photo-detector for making a photometric determination regarding the contents of the sample, the improved method allowing a photometric measurement to be made after a reduced warm-up period and compensating for variations in the intensity of the source during the measurement, the improved method comprising the steps of: turning on the light source;   causing the light source intensity to asymptotically approach a final stable value in a smooth and substantially exponential curve;   processing outputs of the reference and sample photodetectors taken at different times while the source intensity approaches a final stable value and compensating for variations in the source intensity over the time period during which the outputs are taken;   wherein the processing of outputs of the reference and sample photodetectors includes: processing a series of four measurements taken at different times, a first pair of measurements being the outputs of the reference photodetector taken at equal times before and after a midpoint in time, the second pair of measurements being the outputs of the sample photodetector taken at equal times before and after the midpoint,   averaging the first pair of measurements to produce an average reference photodetector measurement and averaging the second pair of measurements to produce an average sample photodetector measurement, and,   taking a ratio of the average sample photodetector measurement and the average reference photodetector measurement to determine the absorbance characteristic of the sample as if the sample and reference photodetector measurements were taken at the same midpoint in time.